The Center for Equine Health and the Bernard and Gloria
Salick Equine Viral Disease Laboratory took a proactive step
in the fight against emerging viral diseases on May 16, 2000.
Dr. Gregory Ferraro, CEH Director, and Dr. James MacLachlan,
Director of the Bernard and Gloria Salick Equine Viral Disease
Laboratory, hosted a meeting on West Nile fever with regulatory
officials
from both the United States Department of Agriculture (USDA)
and the California Department of Food and Agriculture (CDFA),
horse industry representatives from various breeds and disciplines,
and scientists with expertise in equine virology. The informational
seminar provided a forum to discuss the implications of West
Nile fever on the California equine industry and to help prepare
for a possible outbreak this summer. Last August through October,
the West Nile virus caused illness and death in horses, humans
and birds in New York. The virus was also identified in wild
birds in Connecticut and New Jersey. Prior to this outbreak,
the West Nile virus had never been documented in the Western
Hemisphere. Veterinary pathologists at the Bronx Zoo noted a
large number of crows dying near the zoo and examined the birds.
Shortly after, people in northern Queens, just across the East
River from the zoo, were reported with encephalitis-like infections
and some of the zoo birds suddenly died. Twenty five horses were
infected in one location on Long Island and eight of those animals
died. The culprit was eventually identified as the West Nile
virus, an encephalitis virus.
West Nile virus was originally discovered in 1937 in Uganda.
Subsequently, it has been recognized throughout Africa, the Middle
East and Eastern Europe. This virus was not known to exist in
the Western Hemisphere until the New York outbreak last summer.
Considering the historical record for how viruses spread once
they are established in a geographical area, North Americans
should expect the disease to become an endemic problem for them
as well. This winter, scientists identified the virus in mosquitoes
and in a dead red tailed hawk in New York which proves that the
virus has over-wintered in New York. However, given the migratory
patterns of birds within the Western Hemisphere, the virus could
re-appear anywhere in the United States this summer.
West Nile virus is categorized within the Japanese encephalitis
group of viruses. This group includes many viruses already endemic
in California such as western equine encephalomyelitis (WEE),
Venezuelan equine encephalomyelitis (VEE) and St. Louis encephalitis.
West Nile virus is very closely related to the St. Louis virus;
both of them are designated scientifically as flaviviruses. The
virus' natural hosts are birds that migrate between the northern
and southern hemispheres, and the virus is transmitted between
birds by mosquitoes. Horses, humans and other susceptible animals
are considered "accidental " or "dead-end"
hosts that become infected after being bitten by infected mosquitoes.
While the virus causes illness in these accidental hosts, most
infected birds do not become ill, thereby making them excellent
carriers of the disease.
The disease causes inflammation of the
central nervous system, but the clinical symptoms for West Nile
fever in the horse and in man often are not diagnostic. They
include typical encephalitic symptoms such as disorientation,
dementia and coma. In the horse, the disease can present as a
weakness or ataxia of the hind limbs. The viremiaand fever that
occur following initial infection in the horse often precede
the neurological symptoms by several weeks; therefore, the disease
can be confusing and difficult to diagnose in the horse. There
is no specific treatment for West Nile fever other than lessening
the severity of central nervous system disturbance. Currently,
there is no vaccine for West Nile fever and routine vaccination
against western and eastern equine encephalomyelitis does not
confer protection against West Nile fever.
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CEH Prepares for West
Nile Fever Outbreak
